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Pulsars are the most energetic stellar objects in the Universe.
We think of them as cosmic lighthouses sweeping twin beams of radiation as they rotate up
to hundreds of times a second.
These beams emerge from the magnetic poles which are thought to lie in opposing hemispheres
of the star.
But astronomers using NASA’s NICER telescope created the first-ever maps of a pulsar 1100
light-years away.
They found that magnetic fields of pulsars are even more weird than previously thought.
Welcome back to Launch Pad, I'm Christian Ready, your friendly neighborhood astronomer.
In our previous video Neutron Stars, Pulsars, and Magnetars, we learned that they form when
the cores of massive stars collapse, setting off a supernova.
These objects are so mind-bogglingly dense, protons and electrons are squeezed together
to create neutrons.
A neutron star is essentially a giant atomic nucleus more massive than the Sun, yet squeezed
into a sphere the size of a city.
Understanding the conditions inside neutron stars is one of the greatest mysteries in
physics.
Their interiors are thought to consist of exotic states of superfluid neutrons and superconducting
protons, but the most extreme conditions in the core remain unknown.
To understand them requires pushing the boundaries of our understanding of quantum mechanics.
Quantum mechanics is a fascinating but mind-bending subject.
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Neutron stars rotate ridiculously fast, up to hundreds of times a second.
This creates ultra-powerful magnetic fields that are so strong, they actually rip particles
off the star's surface.
These particles race along the magnetic field lines until they slam back onto the surface
at the magnetic poles.
This heats up the poles so much they give off X-rays.
And these X-ray hotspots mark the launch points of the twin beams of radiation.
Typically the magnetic axis is tilted with respect to the neutron star’s rotation axis,
so the beams of radiation rotate with the star like a cosmic lighthouse.
When the beams sweep along our line of sight we detect them as pulsars.
At least, that’s the canonical model of pulsars as rotating magnetic dipoles.
Or put another way, super-spinning power zombie bar magnets.
But much about neutron stars remains enshrouded in mystery, especially their interiors.
To that end, the Neutron star Interior Composition Explorer, or NICER, was installed on the International
Space Station in 2017.
NICER uses an array of 56 detectors to record the precise energy levels and arrival time
sof X-ray photons to an accuracy of just 100 nanoseconds.
That’s 20 times more accurate than previous X-ray timing observatories.
Astronomers can use this information to build up a map of the X-ray hotspots on a pulsar,
and then work backwards to model its interior.
This is a technique called Pulse-Profile Modeling and in a way it's similar to how geologists
use seismic data to model the interior of Earth.
But in the case of a pulsar, one must also account for things like how the temperature
varies across the star’s surface, how radiation is transferred in the stellar atmosphere,
and how light-rays from the pulsar get distorted in its strong gravity.
Pulsars are so dense, their gravity warps the fabric of space-time around them.
This has the effect of letting us see parts of the pulsar’s far side.
To see what I mean, imagine a pulsar about 8 miles in radius, but with far less mass.
We'd see the object at its actual size, and only see light coming from the near side of
the pulsar.
But if we let the mass increase, the pulsar’s gravity warps space-time and bends light from
the far side toward us.
The more massive the pulsar gets, the more it warps space-time, and the more of the far
side we can see!
This means the hot spots may never quite disappear from view.
But the light from the far side would be slightly delayed as it has to travel a longer distance
toward us.
This creates a subtle difference in the timing of the arriving X-rays.
Astronomers can take advantage of this effect to infer things like the pulsar’s mass,
radius, and the location of the hotspots.
From 2017 to 2018, NICER examined the pulsar PSR J0030+0451.
J0030 is an isolated pulsar 1100 about light-years away in the constellation Pices.
Its rotation period is just 4.87 milliseoconds.
In other words, it rotates more than 205 times a second!
This puts J0030 in a special class of fast-rotating “millisecond pulsars”.
As J0030 spun, its X-ray flux was precisely measured by NICER.
Two teams of astronomers worked independently to infer the pulsar’s properties from the
NICER data.
It turns out, both teams got very similar results.
One team based in the United States.
estimated the pulsar at 1.34 solar masses and 12.71 km in radius.
A second team based in the Netherlands got a similar result of 1.44 solar masses and
a radius of 13.02 km.
These are the most precise measurements yet of a pulsar’s mass and size with an uncertainty
of less than 10%.
That puts J0030 toward the lower end of pulsar masses, which are thought to range between
1.1 and 2.16 solar masses.
By the way, a little side note here.
In our previous video on neutron stars, we talked how they form from the collapse of
a massive star when its white dwarf core reaches 1.4 solar masses.
This is the Chandrasekhar Limit, above which electron degeneracy pressure can no longer
hold the core up against its own gravity, and the core collapses to form a neutron star
or even a black hole.
So how can a neutron star have a mass lower than the Chandrasekhar Limit?
After it forms, the surrounding layers of the dying star slam down onto the core’s
surface, setting off the supernova.
But when all of that mass collides onto the neutron star, a significant amount gets converted
into pure energy in accordance with Einstein’s E = mc^2.
Thus, a significant fraction of the core’s mass gets annihilated, and the resulting neutron
star can weigh less than the Chandrasekhar Limit.
To that end, the teams not only weighed the pulsar but they also determined that we’re
viewing the pulsar from above its north rotational pole at an inclination of about 54 degrees.
But when it came to mapping the hotspots, the data didn’t fit the traditional model
of a magnetic dipole.
Instead, both teams found that both hotspots were coming from the pulsar’s Southern Hemisphere!
It gets even weirder than that.
The team led by researchers at the University of Amsterdam suggests that J0030 has one small
circular spot and an elongated crescent-shaped spot both surrounding the south rotational
pole.
The other team, led by researchers at the Universities of Maryland and Illinois, got
a similar result, with two oval hotspots on either side of the South Pole.
Now these spots have similar shapes and locations as the ones found by the Dutch team.
But the American team found a third, cooler spot just slightly offset from the pulsar’s
South Pole.
This third spot was barely detected just at the edge of our view of the pulsar.
Both are extremely weird results, and look nothing like the classical magnetic dipole
model.
So does this mean our understanding of pulsars is fundamentally wrong?
Well, researchers who study pulsars would be the first to tell you that the traditional
dipole model hasn’t satisfactorily explained all of the observed pulsar phenomena over
the last 50 plus years.
Higher-order magnetic multipoles had been proposed to explain different pulsar observations.
But the resulting magnetic fields are much more complex and require numerical simulations
that can only be done on supercomputers.
However, the NICER data revealed two distinct repeating pulse components, so both teams
were able to test models with two hot regions on the star’s surface while varying the
location and shape to include ovals and crescents.
The fact that both results are similar to each other strongly suggests that there’s
a real non-canonical magnetic dipole on J0030.
But everything else about the pulsar, its timing behavior, gamma-ray output, its X-ray
properties and radio pulsations, are all typical for millisecond pulsars.
And that suggests that what we’re seeing in J0030 might not be all that unusual for
millisecond pulsars.
And that has some interesting implications.
Perhaps the pulsar started off with a typical dipole that somehow got shunted way off from
the center down to the pulsar’s southern hemisphere.
Such offset magnetic fields are not uncommon in nature.
For example, planets Uranus and Neptune in our own Solar System have offset magnetic
fields as well.
Another possibility is that J0030 is undergoing a change in its magnetic field.
Pulsars do evolve over time, often slowing down as their magnetic fields drag against
the interstellar medium.
Although how it results in the magnetic field implied by these results isn’t exactly clear.
Perhaps its polarity is in the process of switching.
The Sun undergoes a polarity reversal during every 11-year sunspot cycle.
Perhaps J0030 is undergoing some sort of temporary magnetic realignment.
Now J0030 is just the first pulsar to be modeled to this level of detail.
NICER is investigating other pulsars and it will be interesting to see how their hotspot
and magnetic configurations compare.
Over time, a better picture of the evolution of pulsars will emerge, along with a better
understanding their interiors.
If you want to learn the difference between neutron stars, pulsars, millisecond pulsars,
and magnetars, I invite you to check out this video when we’re done here.
I’d also like to thank my Patreon supporters for helping to keep this channel going, and
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And special thanks as always to my cosmological patron Michael Dowling.
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In the meantime, stay home, stay healthy, and stay curious, my friends.